Implement page.waitForFunction method

The page.waitForFunction method allows to wait for a general predicate.
The predicate will be continiously polled for in page, until
it either returns true or the timeout happens.

The polling parameter could be one of the following:
- 'raf' - to poll on every animation frame
- 'mutation' - to poll on every dom mutation
- <number> - to poll every X milliseconds

References #91

lib/Page.js

@@ -571,6 +571,16 @@ class Page extends EventEmitter {
     return this.mainFrame().waitForSelector(selector, options);
   }
 
+  /**
+   * @param {function()} pageFunction
+   * @param {!Object=} options
+   * @param {!Array<*>} args
+   * @return {!Promise}
+   */
+  waitForFunction(pageFunction, options = {}, ...args) {
+    return this.mainFrame().waitForFunction(pageFunction, options, ...args);
+  }
+
   /**
    * @param {string} selector
    * @param {!Array<string>} filePaths
@@ -637,10 +647,10 @@ function convertPrintParameterToInches(parameter) {
   if (typeof parameter === 'undefined')
     return undefined;
   let pixels;
-  if (typeof parameter === 'number') {
+  if (helper.isNumber(parameter)) {
     // Treat numbers as pixel values to be aligned with phantom's paperSize.
     pixels = /** @type {number} */ (parameter);
-  } else if (typeof parameter === 'string') {
+  } else if (helper.isString(parameter)) {
     let text = parameter;
     let unit = text.substring(text.length - 2).toLowerCase();
     let valueText = '';